Polycrystalline (poly-Si) thin films are conventionally obtained by plasma deposition, solid phase crystallization (SPC), or liquid phase crystallization. Recent advancements in polycrystalline silicon (poly-Si) thin film technology have reduced the time, annealing temperature and overall costs of fabricating such thin films. For example, U.S. Pat. No. 5,147,826 (Lin et al.) and U.S. Pat. No. 5,275,851 (Fonash et al.) are directed to low temperature crystallization and patterning of a-Si films through the deposition of a nucleating site material on either a substrate or the a-Si film. U.S. Pat. Nos. 5,543,352; 5,585,291; 5,643,826 and 5,654,203 to Ohtani et al. are directed a method for manufacturing semiconductor devices with a crystalline silicon layer. U.S. Pat. Nos. 5,543,352, 5,643,826 and 5,654,203 describe the deposition of a solution containing a catalyst in contact with an a-Si film and the crystallization of the a-Si film at a relatively low temperature. U.S. Pat. No. 5,585,291 describes a crystallization method in which a crystallization promoting material is mixed within a liquid precursor material for forming silicon oxide and the precursor material is then coated onto an amorphous silicon film. These advancements in poly-Si technology have made it more cost effective to produce poly-Si films.
Polycrystalline silicon is known to be used in a variety of applications and technologies, such as in infra-red filters, absorbers in solar cells, active mechanical parts in microelectromechanical systems (MEMS), channel layers in transistors, and active layers in sensor structures. In these applications, the poly-Si is in thin film form, e.g., the film is deposited on some substrate in thickness ranging from tens of nanometers (nm) to micrometers (.mu.m). In all these applications, the optical, mechanical or electrical properties or perhaps all three of the poly-Si are being used. Depending on the application, it can be very advantageous to tailor the properties of the thin film poly-Si.
For instance, amorphous silicon (a-Si) is currently known to be used to fabricate large area and low cost systems, such as solar cells and flat panel displays. However, it is possible to employ poly-Si thin films to fabricate three dimensional (3-D) microelectronics and large area, relatively inexpensive electronic systems, at low processing temperatures. That is, poly-Si films provide even higher carrier mobility, doping efficiency and stability, than a-Si. Thus, poly-Si thin films can be used in an increasing number of industries and applications, provided that such thin films can be fabricated with suitable material properties.
The majority of solar cells are presently based on bulk cast poly-Si, which is expensive to produce. A major problem with using poly-Si thin films (instead of bulk cast poly-Si) in solar cells is their low efficiency in absorbing light, since crystalline silicon is an indirect bandgap semiconductor. As such, bulk single crystalline or bulk cast poly-Si solar cells, typically hundreds of microns thick, must be employed to absorb most of the sunlight. However, employing thicker poly-Si thin films will obviate the cost advantage provided by the thin film approach. Moreover, a poly-Si thin film exhibits more defects than bulk poly-Si due to its smaller grain size and denser intra-grain defects. The higher defect density of poly-Si thin film causes a shortened minority carrier diffusion or drift length for the collection of photo-generated carriers. Consequently, if thin film poly-Si solar cells are to be efficient, they must be thin enough (e.g., tens of microns or less) for effective photo-carrier collection and must absorb sunlight, thus requiring some type of light trapping capability.
One approach to enhance the amount of light absorption in a thin film poly-Si cell is to utilize a textured surface. However, such an approach is costly, as it requires additional processing steps and processing time.
Alternatively, enhanced absorption properties of as-deposited microcrystalline Si have been shown to be advantageous in solar cells by the researchers from Institute de Microtechnique, Neuchatel, Switzerland (J. Meier et al., Mat. Res. Soc. Symp. Proc., 420, 3 (1996)). However, the morphology of their hydrogenated microcrystalline material is assumed to be a composition of crystalline grains (160 to 170 .ANG.) embedded in a matrix of a-Si. Since a-Si acts like a direct bandgap semiconductor, it has a much stronger absorption above the bandgap than crystalline silicon. Thus, enhanced absorption behavior of the as-deposited microcrystalline silicon was not (and could not be) attributed in those works to the crystalline phase alone. Furthermore, a correlation with grain size was not reported, and the enhanced light absorption was considered to be due to light scattering from the grain boundaries.
Accordingly, there is a need to provide a method for systematically increasing the optical absorption properties of poly-Si thin films. There is a further need to provide a method for selectively controlling and adjusting the optical absorption properties of poly-Si thin films.
Furthermore, there is an increasing need to enhance the mechanical and electrical properties of poly-Si thin films. Poly-Si thin films are conventionally obtained by plasma deposition, solid phase crystallization (SPC), or liquid phase crystallization. Plasma deposition of thin film poly-Si yields a low density of defects due to in situ hydrogen passivation during the process. However, poly-Si films deposited by plasma enhanced chemical vapor deposition (PECVD) were found to posses smaller grain sizes and certain amorphous content, which limit carrier mobility (J. Meier et al., Mat. Res. Soc. Symp. Proc., 420, 3 (1996)). Unlike for a-Si films, the deposition rate for poly-Si films is very low. Although SPC yields the largest grain sizes and the highest crystallinity, the presence of high density of intra-grain defects in SPC poly-Si limits its use. The intra-grain defects can be passivated by a hydrogen plasma exposure after crystallization; however, post hydrogenation was found to lead to electronic instability (V. Suntharalingam et al., Appl. Phys. Lett., 68, 1400 (1996)). Thus, there is a need to provide a method for controllably adjusting and enhancing the mechanical and electrical properties of crystalline films.
Accordingly, it is an object of the present invention to provide a method for tailoring the material properties of crystalline thin films, such as their optical, mechanical and electrical properties.
It is a further object of the present invention to provide a method for controllably obtaining desired grain sizes for crystalline thin films.
It is also an object of the present invention to provide a method for controllably adjusting the optical absorption properties of crystalline films and, more specifically, for controllably enhancing the optical absorption properties of such films.
Another object of the present invention is to provide a method for controllably adjusting and enhancing the mechanical properties (e.g., stress state or stress formation levels) of crystalline thin films.
It is another object of the present invention to provide a method for controllably adjusting and enhancing the electrical properties of crystalline thin films, such as enhancing their doping efficiencies, carrier mobility, control of the Fermi level and minority carrier lifetimes carrier mobility.